The thickeners usually used in cosmetics or pharmaceuticals to control the rheology of compositions, especially aqueous compositions, generally undergo a reduction in viscosity when the temperature of the medium increases.
However, this behavior can present certain drawbacks, such as the change in the rheology of the composition as a function of changes in temperature (compositions which become fluid in summer and are more viscous in winter).
Particular polymers whose solubility in water is modified above a certain temperature are known in the prior art. These are polymers with a heat-induced demixing temperature (or cloud point) thus defining their region of water solubility. The minimum demixing temperature obtained as a function of the polymer concentration is known as the “LCST” (Lower Critical Solution Temperature).
Some of these polymers are especially described in the articles by Taylor et al., Journal of Polymer Science, part A; Polymer Chemistry, 1975, 13, 2551; by Bailey et al., Journal of Applied Polymer Science, 1959, 1, 56; and by Heskins et al., Journal of Macromolecular Science, Chemistry A2, 1968, 1441.
According to the teaching of patent applications EP-A-0583814 and EP-A-0629649, certain polymers having a critical temperature of the LCST type are used as thermoreversible viscosity-promoting additives in the manufacture of fluids or lubricants used in numerous industrial sectors.
Moreover, it is known, especially from patent application WO-95/24430, to use such heat-sensitive and pH-sensitive polymers in cosmetics or pharmaceuticals. The polymers described in said patent application may be of any chemical nature; in particular, they may be in the form of grafted copolymers comprising a pH-sensitive backbone with heat-sensitive grafts; or, conversely, in the form of a heat-sensitive backbone bearing pH-sensitive grafts; or alternatively in the form of block copolymers formed from pH-sensitive units and heat-sensitive units.
The heat-sensitive grafts or blocks therefore possess an LCST-type temperature as defined above. These blocks or grafts may be prepared by polymerization of vinyl monomers or by polymerization of cyclic ether monomers. In particular, these grafts or blocks may be in the form of poly(substituted N-alkyl)acrylamides or of block copolymers of ethylene oxide and of propylene oxide.
However, the heat-induced gelling properties of these polymers lead to opaque gels, as is pointed out in the description of said patent application. Now, this opacification of the aqueous solutions may be an unacceptable drawback for uses in cosmetics.
This is likewise the case with respect to patent application WO-97/00275, which describes in many cosmetic applications the use of polymers with heat-sensitive units, that are only in the form of block copolymers of ethylene oxide and of propylene oxide. Moreover, in said patent application, it is not easy to control the structure and the chemical nature of the synthesized polymers.